AppKit cursorRects does not fall back to NSTrackingArea based cursorUpdate when popping last cursor

Number:rdar://34183708 Date Originated:
Status:Open Resolved:
Product:AppKit Product Version:10.12.6
Classification:Bug Reproducible:Always
There's two APIs to do cursor handling in AppKit today:

 - The legacy cursorRect approach
 - The new NSTrackingArea based approach

-[NSApplication _handleCursorRectEvent:] calls [[NSCursor arrowCursor] set] when the cursor stack is emptied, instead of falling back to allowing the event to propagate into a -[NSView cursorUpdate:] .

Steps to Reproduce:
This can be seen when for example putting a NSTextField (which still uses the legacy API), into a NSView with an NSTrackingArea and cursorUpdate overriden to set a hand cursor e.g.. 

-[NSView cursorUpdate:] is called when the cursor enters the root view, as expected, and NSDebugCursorRects shows that a cursor rect is pushed onto the stack when entering the text field and changing the cursor into an I-bream, but when the cursor then leaves the text field, the cursor rect is popped, and the arrow cursor is set, never calling -[NSView cursorUpdate:].

Expected Results:
Moving the mouse from the text field to the root view restores the root view's cursor via -[NSView cursorUpdate:]

Observed Results:
The cursor is reset to the arrow cursor.


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